左連線vs左外連線

In SQL, joins are used for the combination of records coming from different sets of data.  The 參加 can either be an inner join or an outer join.  An inner join returns records which have matches in both tables as opposed to an outer join which is the opposite of the inner. The outer join therefore returns those records that do not have matches in either table. Differences between the right outer join and the left outer join are discussed below.

差異

As noted earlier, an inner join will only produce a set of records that is present in two tables that are being compared.  A full outer join, on the other hand, is one that produces the complete set of all the records present in both tables under comparison.  In the event that there are no matches, the missing matches will contain a null.

A left outer join will contain a full set of records that come from the first table and the matching results will be available with its results in the corresponding table. In the event there are no matching results, the right side will contain a null. To produce the records in the left table only, and not the right table,  use of a ‘where’ clause comes into play.

To produce unique records for the table on the right and the one on the left, use of a full outer join is recommended. Having performed the full outer join, a “where” clause is used to exclude results that are unwanted both from the “right” and the “left” side. Furthermore, a Cartesian join can be used to help join everything left and right. This may at times not be what is being sought for, but is what sometimes appears. The joins produce a powerful data set which gives up to 16 rows of data sets, often much more than is anticipated.  Although you get a massive amount of data sets, these joins are extremely dangerous as a slight incompatibility can cripple the entire system.

If you are dealing with a project that also seeks the compatibility of Microsoft SQL server, then there will be benefits derived from the use of the left outer join. The return sequence starts off with inner records being returned first, followed by right join records and finally a join of the left records. Use of the left join or left outer join statement in the SQL environment will refer to the exact same statement. This in essence means that there is no difference as to the result expected whether a left join is used or a left outer join is used. The result will be similar, unless the environment is in the Microsoft SQL server. The outer keyword can be used or even omitted without 擔心 as the results do not differ in any way.

摘要

Joins are used in SQL to compare different sets of data

An inner join only produces a set of records that is present in two tables that are being compared

An outer join produces the complete set of all the records present in both tables under study

A ‘where’ clause used to produce unique records after use of a full join

A Cartesian join links both left and right elements of a table.

There is a notable difference where left join and left outer join used when using Microsoft SQL server

All in all, use of the left join or left outer join statement refers to the exact same statement. Since there is no difference, the use of the left outer join is recommended.